Intestinal gas is a normal byproduct of digestion. You begin producing it shortly after you're born and can even continue producing it after you die.

Everyone's familiar with the consequences of excess intestinal gas: burping, bloating, flatulence and, sometimes, social embarrassment. But how much do you know about the source of this distress: gas itself?

Intestinal gas became a hot research topic during the mid-20th century. Because gas expands at extreme altitudes, sometimes causing painful cramps, scientists studied ways to reduce gas formation in military pilots and astronauts. Their studies, along with those of later researchers, yielded answers to many common questions about flatulence and helped identify some everyday solutions to the problem. Here's the lowdown, along with practical tips.

More than 99 percent of intestinal gas is a mixture of the same odorless gases that are present throughout the environment. These include nitrogen, oxygen, carbon dioxide, hydrogen and sometimes methane. But the exact composition of this mixture varies from person to person, depending on body chemistry and diet.

The nitrogen, oxygen and most of the carbon dioxide in gas come from the air you swallow when you eat, drink, chew gum or smoke. Chemical reactions in your stomach also produce some of the carbon dioxide content of intestinal gas.

Bacteria normally present in the large intestine produce the hydrogen and methane in intestinal gas. These bacteria feed on undigested food particles and release gases during a process called fermentation.

Trace gases — which make up less than 1 percent of the intestinal gas you produce — are responsible for 100 percent of the smell of intestinal gas. The most common trace gases produced by bacteria are hydrogen sulfide, which smells like rotten eggs; methanethiol, which smells like decomposing vegetables; and dimethyl sulphide, which smells sweet.

The most offensive intestinal gases usually contain sulfur compounds from sulfur-rich foods, such as cauliflower, eggs and meat. Although less-sulfur-rich foods such as beans can produce large amounts of gas, such gas has a less noticeable odor.

The expulsion of intestinal gas makes the anal sphincter vibrate, resulting in the characteristic sound. The amount of gas and the force of the expulsion, as well as the amount of resistance from the anal sphincter, determine the loudness and pitch.

Carbonated beverages are an important source of intestinal gas. So are carbohydrates because they often contain indigestible sugars, starches and fiber. For example:

• Raffinose. This sugar is found in beans, cabbage, brussels sprouts, broccoli, asparagus and whole grains.
• Lactose. Milk and milk products such as cheese and ice cream, as well as some processed breads, cereals and salad dressings, contain the sugar lactose. While most people can digest lactose with no difficulty, some have trouble because they do not produce the enzyme lactase, which splits lactose into digestible parts. Without lactase, milk and other lactose-rich foods ferment in the intestine, releasing excessive gas. People of African, Asian and American Indian descent are most likely to be deficient in lactase.
• Fructose. Onions, artichokes, pears, wheat, and some soft drinks and processed foods contain this sugar, which may be difficult to digest.
• Sorbitol. Apples, pears, peaches, prunes, and some sugar-free foods, candies and chewing gum contain the sugar sorbitol, another one that's hard to digest.

Rice is the only starch absorbed almost completely by the small intestine. Because rice starch never reaches the large intestine, gas-producing bacteria don't break this starch down. Other starches, including potatoes, corn, noodles and wheat, are gas producers. Dietary fiber, found in beans, oat bran, peas and most fruits, also tends to produce gas. When research subjects ate a diet in which half of their calories came from pork and beans, they experienced a tenfold increase in their normal gas production.

Yes. The average adult produces one to three pints of intestinal gas a day and passes gas 14 to 23 times a day. Vegetarians, carbonated-beverage drinkers, gum-chewers and smokers tend to produce more than the average amount of intestinal gas, as do people who eat lots of gas-producing foods.

Two intestinal gases — hydrogen and methane — are combustible and potentially explosive.

Not everyone produces these two gases. The concentration of hydrogen in intestinal gas can range from zero to 86 percent while the concentration of methane can range from zero to 56 percent. Only about a third of the population produces methane, a trait that appears to be inherited.

Although it's not dangerous to pass gas while working near an open flame, rare instances of gas explosion during intestinal surgery have been reported.

Probably not. When you hold in gas, it migrates backward through your gastrointestinal tract. If gas becomes trapped in one of the areas where your colon bends sharply — in the upper right and upper left parts of the abdomen — it can cause pain. This pain should subside when you release the gas — as you eventually must, because the body does not reabsorb it.

The most common ways to reduce intestinal gas are to change your diet, avoid swallowing air and take over-the-counter (OTC) medications.

Change your diet
If you're deficient in lactase, the enzyme needed to digest milk, your doctor may suggest that you eliminate dairy products from your diet and get the calcium you need from calcium supplements. Otherwise, there's probably no need for you to stop eating entire classes of food. Instead, try cutting out or reducing your intake of a few gas-producing foods for a week or two. If your symptoms improve, your problem is solved.

Swallow less air
To swallow less air — which makes its way to your intestine — don't suck on hard candy or chew gum, and steer clear of carbonated beverages. Don't drink through a straw because this causes you to swallow more air. Of all the reasons you shouldn't smoke, the fact that you swallow air and get gas from smoking is among the least compelling. Still, quitting smoking may be beneficial if you're troubled by gas.

Take OTC gas-reducing medication
Products that contain antacids and simethicone, a foaming agent that pulls gas bubbles together (Mylanta, Di-Gel, others) have no effect on intestinal gas. They may, however, help you belch away excess stomach gas. The jury is still out on anti-gas medications containing activated charcoal (Nature's Way Activated Charcoal, CharcoCaps, others). If they do work, their effects aren't dramatic.

The digestive enzyme lactase (Lactaid and Lactrase) may relieve excess gas caused by dairy products. Another digestive enzyme, alpha-galactosidase (Beano), may reduce the gas caused by beans and other carbohydrates containing the sugar raffinose.

Over-the-counter medications containing bismuth subsalicylate (Pepto-Bismol, others) reduce the odor of one of the most offensive intestinal gases, hydrogen sulfide. Unfortunately, you can't take such medications for more than a few days because a buildup of bismuth can be toxic.

A more practical solution may be to wear or sit on a product containing activated charcoal, which helps absorb malodorous gas. Such products include undershorts, pads worn inside undershorts and seat cushions. Of the three, undershorts may be the most effective.

By itself, intestinal gas is rarely a sign or symptom of a serious condition. But you should see your doctor if your gas is persistent or severe, or if it's associated with vomiting, diarrhea, constipation, unintentional weight loss, blood in the stool or heartburn. Excessive gas may be a sign of another digestive disorder such as celiac disease, gastritis, gastroesophageal reflux disease (GERD) or irritable bowel syndrome.

Intestinal gas can cause discomfort and embarrassment, but it's usually just a sign of a normally functioning digestive system. If you're bothered by excessive gas, try changing your diet. See your doctor if your gas is severe, persistent or accompanied by other signs and symptoms such as vomiting, diarrhea or constipation.